Plated steel sheet

Abstract

Provided is a plated steel sheet characterized by comprising a base-material steel sheet, and a plating layer formed on a surface of the base-material steel sheet, wherein: the plating layer has a prescribed chemical composition; in a cross section of the plating layer, the length L of the interface between the plating layer and the base-material steel sheet and the length L0 of the surface of the base-material steel sheet satisfy (L-L0) / L0×100 ≥ 3; the plating layer includes an Fe-Al phase; and the thickness of the Fe-Al phase is 4-50 μm.

Description

Plated steel sheet

[0001] The present invention relates to a plated steel sheet.

[0002] Zn-based plated steel sheets are known to exhibit sacrificial corrosion protection and have excellent corrosion resistance. On the other hand, many proposals have been made for plated steel sheets having a plating layer containing other elements instead of or in addition to Zn.

[0003] For example, Patent Document 1 discloses a steel sheet comprising: a substrate steel sheet; a first alloy plating layer having a thickness of 3 to 30 μm and having a composition containing, in mass %, 40 to 70% Fe, 0.3 to 10% Mn, and the balance being Al and unavoidable impurities, on at least one surface of the substrate steel sheet; a second alloy plating layer having a thickness of 0.10 to 10 μm and having a composition containing, in mass %, 5 to 50% Fe, 5 to 40% Mn, and the balance being Al and unavoidable impurities, on the first alloy plating layer; and a second alloy plating layer having a thickness of 0.10 to 10 μm and having a composition containing, in mass %, 5 to 50% Fe, 5 to 40% Mn, and the balance being Al and unavoidable impurities, on the surface of the second alloy plating layer, in a coating amount of 0 to 1000 mg / m 2 Patent Document 1 also describes a method for forming an Al-Fe-Mn alloy plating layer having two layers, each with a different Mn content, on the surface of a substrate steel sheet by hot-dip plating an Al-Mn alloy on the substrate steel sheet, and a method for forming an Al-Fe-Mn alloy plating layer having two layers, each with a different Mn content, on the surface of the substrate steel sheet, and a method for forming an Al-Fe-Mn alloy plating layer having a coating weight of 1000 mg / m 2 It is taught that by limiting the content to the following ranges, it is possible to achieve both corrosion resistance after painting and resistance spot weldability in an environment similar to the corrosive environment of an automobile exterior panel.

[0004] Japanese Patent Application Laid-Open No. 2020-122205

[0005] In general, an alloying treatment is required for the Al-based plated steel sheet described in Patent Document 1 to ensure corrosion resistance after painting. However, since the alloyed Al-based plating is relatively hard, the cold workability of the plated steel sheet may be reduced.

[0006] Therefore, an object of the present invention is to provide a plated steel sheet having an Al-containing plating layer, which has improved corrosion resistance and cold workability after painting.

[0007] As a result of investigations conducted by the present inventors to achieve the above object, it was found that improved corrosion resistance and cold workability after painting can be achieved by optimizing the chemical composition of the Al-containing plating layer and appropriately controlling the thickness of the Fe—Al phase contained in the Al-containing plating layer and the morphology of the interface between the Al-containing plating layer and the base steel sheet, and thus the present invention was completed.

[0008] The present invention, which has achieved the above object, is as follows. (1) A steel sheet comprising a base steel sheet and a plating layer formed on a surface of the base steel sheet, wherein the plating layer contains, in mass%, Fe: 20.0 to 55.0%, Mg: 0 to 10.0%, Si: 0 to 10.0%, Zn: 0 to 30.0%, and further contains Ni: 0 to 1.000%, Ca: 0 to 4.000%, Sb: 0 to 0.500%, Pb: 0 to 0.500%, Cu: 0 to 1.000%, Sn: 0 to 1.000%, Ti: 0 to 1.000%, Cr: 0 to 1.000%, Nb: 0 to 1.000%, Zr: 0 to 1.000%, Mn: 0 to 1.000%, Mo: 0 to 1.000%, It has a chemical composition containing at least one of Ag: 0 to 1.000%, Li: 0 to 1.000%, La: 0 to 0.500%, Ce: 0 to 0.500%, B: 0 to 0.500%, Y: 0 to 0.500%, Sr: 0 to 0.500%, In: 0 to 0.500%, Co: 0 to 0.500%, Bi: 0 to 0.500%, P: 0 to 0.500%, W: 0 to 0.500%, and V: 0 to 0.500% in a total amount of 5.000% or less, and the balance: 20.0% or more of Al and impurities, (1) A plated steel sheet, characterized in that, in a cross section of the plated layer, the interface length L between the plated layer and the base steel sheet and the surface length L0 of the base steel sheet satisfy (L - L0) / L0 x 100 ≧ 3, the plated layer contains an Fe-Al phase, and the thickness of the Fe-Al phase is 4 to 50 μm. (2) The plated steel sheet according to (1) above, characterized in that (L - L0) / L0 x 100 ≧ 5. (3) The plated steel sheet according to (2) above, characterized in that (L - L0) / L0 x 100 ≧ 7. (4) The plated steel sheet according to any one of (1) to (3) above, characterized in that the Mg content in the plated layer is 0.2% or more. (5) The plated steel sheet according to any one of (1) to (4) above, characterized in that the chemical composition contains, in mass %, Mg: 0.3 to 10.0%, and Si: 0 to 1.0%. (6) The plated steel sheet according to any one of (1) to (5) above, wherein the thickness of the Fe—Al phase is 12 to 50 μm.(7) A projected length T of the Fe—Al—Si phase in the plating layer in a cross section of the plating layer. i and the surface length L0 of the base steel plate is ΣT i / L0×100≦20. i / L0 × 100≦1. (9) The plated steel sheet according to any one of (1) to (8) above, characterized in that the chemical composition contains, in mass%, 0.3 to 10.0% Mg, the plated layer further contains an Mg-containing phase, and the surface coverage of the Mg-containing phase is 20 to 100% in a cross section of the plated layer. (10) The plated steel sheet according to (9) above, characterized in that the surface coverage of the Mg-containing phase is 60 to 100%. (11) The plated steel sheet according to any one of (1) to (10) above, characterized in that the Mg content in the plated layer is 2.4% or less. (12) The plated steel sheet according to any one of (1) to (11) above, characterized in that the Si content in the plated layer is 0.2% or more. (13) The plated steel sheet according to any one of (1) to (12) above, wherein the area ratio of the MgZn2 phase in the plated layer is less than 10%.

[0009] According to the present invention, it is possible to provide a plated steel sheet having an Al-containing plating layer, which has improved corrosion resistance and cold workability after painting.

[0010] 1 is a cross-sectional schematic view of a plated steel sheet according to an embodiment of the present invention, showing the interface length L between the plated layer and the base steel sheet and the surface length L0 of the base steel sheet. i 1 is a cross-sectional schematic view of a plated steel sheet according to another preferred embodiment of the present invention, illustrating the surface coverage of the Mg-containing phase.

[0011] <Plated Steel Sheet> A plated steel sheet according to an embodiment of the present invention includes a base steel sheet and a plating layer formed on a surface of the base steel sheet, wherein the plating layer contains, in mass %, Fe: 20.0 to 55.0%, Mg: 0 to 10.0%, Si: 0 to 10.0%, Zn: 0 to 30.0%, and further contains Ni: 0 to 1.000%, Ca: 0 to 4.000%, Sb: 0 to 0.500%, Pb: 0 to 0.500%, Cu: 0 to 1.000%, Sn: 0 to 1.000%, Ti: 0 to 1.000%, Cr: 0 to 1.000%, Nb: 0 to 1.000%, Zr: 0 to 1.000%, Mn: 0 to 1.000%, It has a chemical composition containing at least one of Mo: 0 to 1.000%, Ag: 0 to 1.000%, Li: 0 to 1.000%, La: 0 to 0.500%, Ce: 0 to 0.500%, B: 0 to 0.500%, Y: 0 to 0.500%, Sr: 0 to 0.500%, In: 0 to 0.500%, Co: 0 to 0.500%, Bi: 0 to 0.500%, P: 0 to 0.500%, W: 0 to 0.500%, and V: 0 to 0.500% in a total amount of 5.000% or less, and the balance: 20.0% or more of Al and impurities, In a cross section of the coating layer, an interface length L between the coating layer and the base steel sheet and a length L0 of the surface of the base steel sheet satisfy (L-L0) / L0×100≧3, the coating layer contains an Fe-Al phase, and the thickness of the Fe-Al phase is 4 to 50 μm.

[0012] As mentioned above, Al-based plated steel sheets generally require alloying treatment to ensure corrosion resistance after painting. However, because the alloyed Al-based plating is relatively hard, the cold workability of the plated steel sheet may be reduced. For example, the alloyed Al-based plating may peel off into powder form (also known as powdering) during cold working, which may reduce powdering resistance. Therefore, it is generally difficult for plated steel sheets having a plating layer made of Al-based plating to achieve both corrosion resistance after painting and cold workability, particularly powdering resistance.

[0013] Therefore, the present inventors conducted research, focusing particularly on the chemical composition, structure, and morphology of the plating layer, in order to achieve both post-painting corrosion resistance and cold workability in a plated steel sheet having a plating layer made of an Al-based plating. As a result, the present inventors found that both post-painting corrosion resistance and cold workability can be significantly improved by optimizing the chemical composition of the plating layer and appropriately controlling the thickness of the Fe—Al phase contained in the plating layer and the morphology of the interface between the plating layer and the base steel sheet.

[0014] More specifically, the present inventors first found that by setting the Fe content in the coating layer to 20.0 mass% or more and controlling the thickness of the Fe—Al phase contained in the coating layer to 4 μm or more, the coating layer can be sufficiently alloyed, thereby improving the corrosion resistance after painting of the coated steel sheet. On the other hand, the present inventors also found that by controlling the thickness of the Fe—Al phase to 50 μm or less, excessive hardening of the coating layer can be suppressed, thereby improving the cold workability of the coated steel sheet.

[0015] Next, the present inventors investigated the morphology of the coating layer in order to further improve the cold workability of the coated steel sheet. As a result, the present inventors discovered that the cold workability of the coated steel sheet can be significantly improved by controlling the interface shape between the coating layer and the base steel sheet to have greater irregularities, more specifically, by controlling the interface shape to have greater irregularities such that the interface length L between the coating layer and the base steel sheet and the surface length L0 of the base steel sheet satisfy the relationship (L - L0) / L0 × 100 ≥ 3. FIG. 1 is a cross-sectional schematic diagram of a coated steel sheet according to an embodiment of the present invention, showing the interface length L between the coating layer and the base steel sheet and the surface length L0 of the base steel sheet. Referring to FIG. 1, the coated steel sheet 1 according to an embodiment of the present invention includes a base steel sheet 2 and a coating layer 3 formed on the surface of the base steel sheet 2, and the coating layer 3 contains an Fe—Al phase 4. In Fig. 1, the interface length L between the coating layer 3 and the base steel sheet 2 and the corresponding length L0 of the surface of the base steel sheet 2 satisfy the relationship (L - L0) / L0 x 100 ≥ 3, i.e., the interface length L is 3% or more longer than the length L0 of the surface of the base steel sheet 2. It can therefore be seen that the interface between the coating layer 3 and the base steel sheet 2 is controlled to have a shape with greater irregularities.

[0016] Without intending to be bound by any particular theory, it is believed that when the interface between the coating layer 3 and the base steel sheet 2 has a shape with greater irregularities as shown in Fig. 1 , the hard coating layer 3 can dig into the base steel sheet 2 from the irregularities at the interface during cold working such as bending, thereby deforming the base steel sheet 2 and allowing the cold working to proceed. As a result, it is possible to significantly suppress the occurrence of powdering due to bending, etc., and in other words, it is possible to significantly improve the cold workability of the coating steel sheet 1. On the other hand, if the interface between the coating layer 3 and the base steel sheet 2 has a flat shape or a flatter shape with less irregularities, it is not possible to allow the hard coating layer 3 to dig into the base steel sheet 2 during cold working such as bending, thereby allowing the cold working to proceed, and therefore it is not possible to sufficiently suppress the occurrence of powdering.

[0017] The inventors have also found that in order to create an interface shape with greater irregularities between the coating layer 3 and the base steel sheet 2, it is effective to increase the alloying rate during the alloying treatment of the coating layer 3. Explaining in more detail, first, excessive Si and Mg content in the coating layer 3 may adversely affect the alloying of the coating layer 3, and therefore the Si and Mg contents in the coating layer 3 must be controlled to 10.0 mass% or less, respectively. In addition, in order to increase the alloying rate of the coating layer 3, it is necessary to appropriately control the metallographic structure of the base steel sheet 2 during the alloying treatment. More specifically, by making the base steel sheet 2 during the alloying treatment have a metallographic structure that is appropriately decarburized and contains a larger amount of austenite phase, the reaction between the coating layer 3 and the austenite phase in the base steel sheet 2 during the alloying treatment is promoted, i.e., the alloying rate can be significantly increased. As will be described in detail later in connection with the manufacturing method of the plated steel sheet 1, the inventors have found that it is possible to create a metallographic structure of the base steel sheet 2 that is moderately decarburized and contains a larger amount of austenite phase by appropriately controlling the annealing step, cooling step, and plating step of the base steel sheet 2. As a result, it is possible to realize an interface shape with greater irregularities such that the interface length L between the plated layer 3 and the base steel sheet 2 and the corresponding length L0 of the surface of the base steel sheet 2 satisfy the relationship (L - L0) / L0 × 100 ≥ 3, thereby making it possible to significantly improve the cold workability of the plated steel sheet 1.

[0018] In particular, the inventors have now revealed for the first time that controlling the Fe—Al phase 4 in an appropriately alloyed coating layer 3 to within a range of 4 to 50 μm ensures sufficient corrosion resistance after painting and improves cold workability, and further that controlling the interface shape between the coating layer 3 and the base steel sheet 2 to a shape with greater irregularities that satisfies the relationship (L−L0) / L0×100≧3 can significantly improve the cold workability of the coated steel sheet 1. Therefore, the coated steel sheet according to the embodiment of the present invention is particularly useful in the automotive field, where both corrosion resistance after painting and cold workability are required.

[0019] Hereinafter, a plated steel sheet according to an embodiment of the present invention will be described in more detail. In the following description, the unit of content of each element, "%," means "mass %" unless otherwise specified. Furthermore, in this specification, unless otherwise specified, the term "to" indicating a numerical range means that the numerical values ​​before and after the range are included as the lower and upper limits.

[0020] [Plating Layer] According to an embodiment of the present invention, a plating layer is formed on the surface of a base steel sheet, for example, on at least one surface, preferably both surfaces, of the base steel sheet. The plating layer has the following chemical composition:

[0021] [Fe: 20.0 to 55.0%] When a plated steel sheet is alloyed, Fe from the base steel sheet diffuses into the plated layer and alloys with Al, etc., so the plated layer inevitably contains Fe. To ensure corrosion resistance after painting, the plated steel sheet must be appropriately alloyed, and therefore the Fe content is set to 20.0% or more. The Fe content may be 25.0% or more, 30.0% or more, 35.0% or more, or 40.0% or more. On the other hand, if the Fe content is too high, excessive alloying of the plated layer may result in a decrease in cold workability. Therefore, the Fe content is set to 55.0% or less. The Fe content may be 52.0% or less, 50.0% or less, 48.0% or less, or 45.0% or less.

[0022] [Mg: 0 to 10.0%] Mg is an element effective in improving the corrosion resistance of the coating layer, particularly its chemical conversion treatability. While the Mg content may be 0%, to achieve this effect, the Mg content is preferably 0.2% or more. The Mg content may be 0.3% or more, 0.5% or more, 0.8% or more, 1.0% or more, 1.5% or more, or 2.0% or more. On the other hand, excessive Mg content may slow the alloying rate during alloying of the coating layer, making it impossible to obtain the desired interface shape between the coating layer and the base steel sheet. Therefore, the Mg content is set to 10.0% or less. The Mg content may be 8.0% or less, 6.0% or less, 5.0% or less, 4.0% or less, 3.0% or less, less than 2.5%, 2.4% or less, or 2.2% or less.

[0023] [Si: 0 to 10.0%] Si is an element effective in improving the adhesion of the coating layer. The Si content may be 0%, but to fully obtain this effect, the Si content is preferably 0.1% or more. The Si content may be 0.2% or more, 0.3% or more, 0.5% or more, 0.6% or more, or 0.8% or more. On the other hand, excessive Si content may slow the alloying rate during alloying treatment of the coating layer, making it impossible to obtain the desired interface shape between the coating layer and the base steel sheet. Therefore, the Si content is set to 10.0% or less. The Si content may be 8.0% or less, 6.0% or less, 4.0% or less, or 2.0% or less. Further reducing the Si content can significantly suppress or reduce the formation of Fe-Al-based intermetallic compounds containing relatively large amounts of Si, more specifically, Fe-Al-Si phases containing 3% or more by mass of Si. If a relatively large amount of the Fe—Al—Si phase is present, bimetallic corrosion (galvanic corrosion) may occur between the Fe—Al phase (content of elements other than Fe, Al, and Zn is less than 3%). Therefore, from the viewpoint of further improving corrosion resistance, the Si content is preferably 1.0% or less.

[0024] [Zn: 0 to 30.0%] Zn has a sacrificial corrosion protection effect and is effective in improving the corrosion resistance of the coating layer. The Zn content may be 0%, but to fully obtain this effect, the Zn content is preferably 1.0% or more. The Zn content may be 3.0% or more, 5.0% or more, 10.0% or more, 12.0% or more, 15.0% or more, or 18.0% or more. On the other hand, excessive Zn content may cause significant Zn melting during welding of the coated steel sheet, and the molten Zn may penetrate the steel and cause liquid metal embrittlement (LME) cracking. Therefore, the Zn content is preferably 30.0% or less. The Zn content may be 28.0% or less, 25.0% or less, 22.0% or less, or 20.0% or less.

[0025] Furthermore, the plating layer may optionally contain Ni: 0 to 1.000%, Ca: 0 to 4.000%, Sb: 0 to 0.500%, Pb: 0 to 0.500%, Cu: 0 to 1.000%, Sn: 0 to 1.000%, Ti: 0 to 1.000%, Cr: 0 to 1.000%, Nb: 0 to 1.000%, Zr: 0 to 1.000%, Mn: 0 to 1.000%, Mo: 0 to 1.000%, Ag: 0 At least one of the following may be contained: Cr: 0-1.000%, Li: 0-1.000%, La: 0-0.500%, Ce: 0-0.500%, B: 0-0.500%, Y: 0-0.500%, Sr: 0-0.500%, In: 0-0.500%, Co: 0-0.500%, Bi: 0-0.500%, P: 0-0.500%, W: 0-0.500%, and V: 0-0.500%. These optional elements are not particularly limited, but preferably their total content is 5.000% or less. The optional elements may have a total content of 4.500% or less, 4.000% or less, 3.500% or less, 3.000% or less, 2.500% or less, 2.000% or less, 1.500% or less, 1.000% or less, 0.800% or less, 0.500% or less, 0.100% or less, or 0.050% or less. These optional elements are described in more detail below.

[0026] [Ni: 0 to 1.000%] Ni is an element effective in improving the corrosion resistance of the plating layer. The Ni content may be 0%, but to achieve this effect, the Ni content is preferably 0.001% or more. The Ni content may be 0.003% or more, 0.005% or more, 0.008% or more, 0.010% or more, or 0.020% or more. While there are no particular upper limits, from the viewpoint of production costs, etc., the Ni content may be 1.000% or less, and may be, for example, 0.500% or less, 0.400% or less, 0.300% or less, 0.100% or less, 0.050% or less, or 0.030% or less.

[0027] [Ca: 0 to 4.000%] Ca is an element effective in ensuring the wettability of the plating bath. The Ca content may be 0%, but to achieve this effect, the Ca content is preferably 0.001% or more. The Ca content may be 0.003% or more, 0.005% or more, or 0.010% or more. On the other hand, excessive Ca content may form a large amount of hard intermetallic compounds in the plating layer, making the plating layer brittle and reducing adhesion to the steel sheet. Therefore, the Ca content is preferably 4.000% or less. The Ca content may be 3.000% or less, 2.000% or less, 1.000% or less, 0.500% or less, 0.300% or less, 0.100% or less, 0.050% or less, or 0.020% or less.

[0028] [Sb: 0-0.500%, Pb: 0-0.500%, Cu: 0-1.000%, Sn: 0-1.000%, Ti: 0-1.000%, Cr: 0-1. 000%, Nb: 0-1.000%, Zr: 0-1.000%, Mn: 0-1.000%, Mo: 0-1.000%, Ag: 0-1.000%, Li: 0-1.000%, La: 0-0.500%, Ce: 0-0.500%, B: 0-0.500%, Y: 0-0.500%, Sr: 0-0.500%, In: 0-0.500%, Co: 0-0.500%, Bi: 0-0.500%, P: 0-0.500%, W: 0-0.500% and V: 0-0.500%] Sb, Pb, Cu, Sn, Ti, Cr, Nb, Zr, Mn, Mo, Ag, Li, La, Ce, B, Y, Sr, In, Co, Bi, P, W and V may not be contained in the plating layer, but may be present in the plating layer in an amount of 0.0001% or more, 0.001% or more, or 0.01% or more. These elements do not adversely affect the performance of the plated steel sheet as long as they are within a predetermined content range. However, excessive contents of each element may reduce corrosion resistance. Therefore, the contents of Sb, Pb, La, Ce, B, Y, Sr, In, Co, Bi, P, W, and V are preferably 0.500% or less, and may be, for example, 0.300% or less, 0.100% or less, 0.050% or less, or 0.020% or less. Similarly, the contents of Cu, Sn, Ti, Cr, Nb, Zr, Mn, Mo, Ag, and Li are preferably 1.000% or less, and may be, for example, 0.800% or less, 0.500% or less, 0.100% or less, 0.050% or less, or 0.020% or less.

[0029] The remainder of the plating layer other than the above elements consists of 20.0% or more of Al and impurities. The Al content may be, for example, 25.0% or more, 30.0% or more, 35.0% or more, 40.0% or more, 45.0% or more, or 50.0% or more. Similarly, the Al content may be, for example, 80.0% or less, 75.0% or less, 70.0% or less, 65.0% or less, or 60.0% or less. Impurities in the plating layer refer to components that are mixed in due to various factors in the manufacturing process, including raw materials, when producing the plating layer.

[0030] [Measurement of Chemical Composition of Plating Layer] The chemical composition of the plating layer is determined as follows. First, the plating layer is stripped and dissolved from the plated steel sheet using an acid solution containing an inhibitor that suppresses corrosion of the base steel sheet, and the resulting acid solution is measured by ICP (inductively coupled plasma) emission spectroscopy to determine the chemical composition (average composition) of the plating layer. The acid species is not particularly limited, and any acid that can dissolve the plating layer may be used. Note that the chemical composition of the plating layer in this embodiment is the average of measurements taken on three samples.

[0031] [(L - L) / L × 100 ≧ 3] In an embodiment of the present invention, the morphology of the coating layer is controlled so that, in the cross section of the coating layer, the interface length L between the coating layer and the base steel sheet and the surface length L of the base steel sheet satisfy the relationship (L - L) / L × 100 ≧ 3, i.e., the interface length L is 3% or more longer than the surface length L of the base steel sheet. As described above in relation to FIG. 1 , by having the interface between the coating layer and the base steel sheet have a shape with greater irregularities that satisfies the relationship (L - L) / L × 100 ≧ 3, during cold working such as bending, the hard coating layer can penetrate into the base steel sheet from the interfacial irregularities, deforming the base steel sheet while the cold working proceeds. As a result, it is possible to significantly suppress the occurrence of powdering due to bending, etc., and, therefore, it is possible to significantly improve the cold workability of the coated steel sheet. To further enhance this effect, it is preferable to control the shape of the interface between the coating layer and the base steel sheet to have larger irregularities, that is, to increase the value of (L - L0) / L0 x 100. More specifically, the value of (L - L0) / L0 x 100 is preferably 4 or greater, and may be, for example, 5 or greater, 6 or greater, 7 or greater, or 8 or greater. There is no particular upper limit, but the value of (L - L0) / L0 x 100 may be, for example, 30 or less, 20 or less, 15 or less, 12 or less, or 10 or less.

[0032] [Fe—Al Phase Thickness: 4 to 50 μm] In an embodiment of the present invention, the coating layer contains an Fe—Al phase, and the thickness of the Fe—Al phase is 4 to 50 μm. In the present invention, the Fe—Al phase refers to a phase having a chemical composition, by mass, of 40 to 70% Fe, 30 to 60% Al, 0 to 20% Zn, and less than 3% other elements (i.e., a total of more than 97% Fe, Al, and Zn). By controlling the Fe content in the coating layer to 20.0 mass% or more, as described above, and the thickness of the Fe—Al phase contained in the coating layer to 4 μm or more, the coating layer can be sufficiently alloyed, thereby improving the corrosion resistance of the coated steel sheet after painting. From the viewpoint of further improving the corrosion resistance after painting, the thicker the Fe—Al phase, the more preferable it is, and it may be, for example, 6 μm or more, 8 μm or more, 10 μm or more, 12 μm or more, 14 μm or more, or 16 μm or more. On the other hand, if the Fe—Al phase is too thick, it may lead to excessive hardening of the plating layer and reduce the cold workability of the plated steel sheet. Therefore, the thickness of the Fe—Al phase is set to 50 μm or less, and may be, for example, 40 μm or less, 30 μm or less, 25 μm or less, or 20 μm or less.

[0033] [Fe-Al-Si phase] [ΣT i / L0 × 100≦20] According to a preferred embodiment of the present invention, in a cross section of the coating layer, the projected length T of the Fe—Al—Si phase in the coating layer i The surface length L0 of the base steel plate is ΣT i / L0×100≦20. The Fe—Al—Si phase is an Fe—Al-based intermetallic compound containing a relatively large amount of Si. More specifically, in the present invention, the Fe—Al—Si phase refers to a phase having a chemical composition consisting of, by mass, 30 to 70% Fe, 30 to 60% Al, 3 to 20% Si, and less than 3% other elements. Therefore, if the Fe—Al—Si phase is present in a relatively large amount in the coating layer, galvanic corrosion may occur between the Fe—Al phase and the coating layer. Therefore, in a preferred embodiment of the present invention, the Fe—Al—Si phase is dispersed in the coating layer, i.e., the projected length T i The surface length L0 of the base steel plate is ΣTi By controlling the content of the coating liquid so as to satisfy the relationship / L0 × 100≦20, it is possible to further improve the corrosion resistance of the plated steel sheet after painting.

[0034] FIG. 2 is a cross-sectional schematic view of a plated steel sheet according to a preferred embodiment of the present invention, showing the projected length T i 2, the plated steel sheet 1 comprises a base steel sheet 2 and a plating layer 3 formed on the surface of the base steel sheet 2, and the plating layer 3 includes an Fe—Al phase 4 and an Fe—Al—Si phase 5. Here, the projected length T i The sum of ΣT i (In Figure 2, ΣT i = T1 + T2), and the surface length L0 of the base steel plate 2 is ΣT i / L0 × 100≦20 (i.e., the projection ratio of the Fe—Al—Si phase is 20% or less), and therefore it can be seen that the Fe—Al—Si phase 5 is dispersed and present in the Fe—Al phase 4. As is clear from the projected length T1 in FIG. 2 , when the projected lengths of multiple Fe—Al—Si phases 5 partially overlap, the entire projected length including the overlapping portions is determined as a single projected length. Unlike the case shown in FIG. 2 , if the Fe—Al—Si phase 5 is present in a layered form in the Fe—Al phase 4, when galvanic corrosion occurs at the contact portion between the layered Fe—Al—Si phase 5 and the Fe—Al phase 4, it is thought that corrosion will progress along the contact interface. As a result, the corrosion resistance of the coating layer 3 will be significantly reduced. In contrast, in the coated steel sheet 1 according to a preferred embodiment of the present invention, ΣT i By dispersing the Fe—Al—Si phases 5 in the Fe—Al phase 4 so as to satisfy / L0×100≦20, even if galvanic corrosion occurs at a contact point between one or more Fe—Al—Si phases 5 and the Fe—Al phases 4 present therearound, the corrosion will not start at the contact point and progress to other Fe—Al—Si phases 5, and therefore the corrosion resistance of the plated steel sheet 1, particularly the corrosion resistance after painting, can be further improved.

[0035] By controlling the Si content in the plating layer, the projection ratio of the Fe-Al-Si phase, ΣT i For example, by controlling the Si content in the plating layer to 1.0% or less, the value of ΣT i / L0 × 100≦20 can be reliably satisfied. i The lower the value of / L0×100, the more preferable, and may be, for example, 15 or less, 10 or less, 5 or less, 3 or less, 2 or less, or 1 or less. From the viewpoint of further improving corrosion resistance, the Fe—Al—Si phase 5 may be prevented from being present in the plating layer. That is, ΣT i The lower limit of / L0×100 may be 0. Although not particularly limited, for example, i The value of / L0 x 100 may be 0.1 or more, 0.2 or more, or 0.3 or more.

[0036] [Surface Coverage of Mg-Containing Phase: 20 to 100%] According to another preferred embodiment of the present invention, the Mg content in the coating layer is 0.3 to 10.0%, and in connection therewith, the coating layer further contains an Mg-containing phase, and the surface coverage of the Mg-containing phase is controlled to 20 to 100% in the cross section of the coating layer. In the present invention, the Mg-containing phase refers to a phase having a chemical composition, in mass %, consisting of 0.5 to 90% Mg, 10 to 99.5% Al, 0 to 70% O, 0 to 3% Fe, and less than 3% other elements. As is clear from this chemical composition, the Mg-containing phase does not include the MgZn2 phase described below.

[0037] Fig. 3 is a cross-sectional schematic view of a plated steel sheet according to another preferred embodiment of the present invention, illustrating the surface coverage of the Mg-containing phase. Referring to Fig. 3, the plated steel sheet 1, like the case of Figs. 1 and 2, comprises a base steel sheet 2 and a plated layer 3 formed on the surface of the base steel sheet 2, and the plated layer 3 contains an Fe—Al phase 4. In Fig. 3, the plated layer 3 further contains an Mg-containing phase 6 in its surface portion. Here, the length M of each Mg-containing phase 6 is i The sum of ΣM i (In Figure 3, ΣM i = M1 + M2 + M3), and the surface length L0 of the base steel plate 2 is ΣM iIt can be seen that the relationship / L0 × 100≧20 is satisfied, i.e., the surface coverage of the Mg-containing phase is 20% or more. By controlling the surface coverage of the Mg-containing phase to 20% or more and allowing a relatively large amount of Mg to be present on the surface of the plating layer, the reaction can be promoted by the action of Mg during chemical conversion treatment, thereby improving the adhesion of the chemical conversion coating to the plated steel sheet. From the viewpoint of further enhancing the effect of improving chemical conversion treatability, a higher surface coverage of the Mg-containing phase is preferable, and may be, for example, 30% or more, 40% or more, 50% or more, 60% or more, 70% or more, or 80% or more. There is no particular upper limit, and the surface coverage of the Mg-containing phase may be 100%. For example, the surface coverage of the Mg-containing phase may be 95% or less or 90% or less. To increase the surface coverage of the Mg-containing phase, it is preferable to increase the Mg content in the plating layer. More specifically, the Mg content in the plating layer is preferably 0.3% or more, and more preferably 0.6% or more. However, since the surface coverage of the Mg-containing phase does not depend only on the Mg content, the Mg content in the plating layer may be appropriately determined according to the desired surface coverage, taking into consideration the manufacturing conditions, etc.

[0038] [Area Fraction of MgZn Phase: Less than 10%] In an embodiment of the present invention, in relation to the upper limit of the Zn content in the plating layer being 30.0%, the MgZn phase may be formed in the plating layer in an area percentage range of less than 10%. The MgZn phase may or may not be present in the plating layer. When the MgZn phase is present in the plating layer, it may contribute to improving sacrificial corrosion protection. The area fraction of the MgZn phase may be, for example, 9% or less, 7% or less, 5% or less, or 3% or less. Similarly, the area fraction of the MgZn phase may be 0%, or, for example, 1% or more, or 2% or more.

[0039] [Analysis of Plating Layer] Analysis of the plating layer is performed as follows. First, a 15 mm x 20 mm sample is taken from the surface of the plated steel sheet so that the cross section of the plating layer can be observed. The sample is embedded in resin and then polished. Next, a backscattered electron image (BSE image) is obtained for the obtained mirror-polished sample using a scanning electron microscope with an electron probe microanalyzer (SEM-EPMA) in a field of view of 80 μm in the thickness direction and 100 μm in the direction perpendicular to the thickness direction, and the plating layer is identified from the BSE image. Next, the composition of each phase in the identified plating layer is analyzed by point analysis. From the obtained composition, the following phases were identified: Fe—Al phase (Fe: 40-70%, Al: 30-60%, Zn: 0-20%, and other elements: less than 3%); Fe—Al—Si phase (Fe: 30-70%, Al: 30-60%, Si: 3-20%, and other elements: less than 3%); Mg-containing phase (Mg: 0.5-90%, Al: 10-99.5%, O: 0-70%, Fe: 0-3%, and other elements: less than 3%); and MgZn2 phase. The specific measurement conditions for the EPMA in the above field of view are as follows: Apparatus: JXA-8500 manufactured by JEOL Ltd. Acceleration voltage: 15 kV Probe current: 5 × 10 -7 A Irradiation time: 50ms

[0040] (L - L0) / L0 x 100 is determined as follows. First, the mirror-polished sample obtained above is observed with an SEM in a field of view of 80 μm in the thickness direction and 100 μm in the direction perpendicular to the thickness direction to obtain a BSE image. The BSE image is then measured using the "Analyze" function of the image analysis software "ImageJ" to measure the interface length between the coating layer and the base steel sheet (interface length L between the coating layer and the base steel sheet shown in Figure 1). The above operation is performed for five fields of view, and the average value is calculated to determine the interface length L. Next, (L - L0) / L0 x 100 is determined from the obtained interface length L and the length L0 of the corresponding base steel sheet surface, i.e., the length of the long side of the observation field: 100 μm. The resolution of the SEM image is 2560 x 1920. To measure L0, use the "Find edge" function in the "Process" section of the image analysis software "ImageJ," binarize the image using the "Binary" function, and then read "Perim." using the "Measure" function in "Analyze."

[0041] The thickness of the Fe—Al phase is determined as follows: First, the thickness of the Fe—Al phase identified above is measured at five different points in the field of view using the “Analyze” function of the image analysis software “ImageJ,” and then the thickness of the Fe—Al phase is determined by averaging the thicknesses measured at the five points.

[0042] ΣT i / L0 × 100 (the projection ratio of the Fe-Al-Si phase) is determined as follows: First, the Fe-Al-Si phases identified above are projected onto the surface of the base steel sheet using the image analysis software "ImageJ", and the projected length T i The sum of ΣT i (In Figure 2, T1 + T2) is calculated. Specifically, using the toolbar "Straight" in ImageJ, a straight line is drawn in the horizontal direction of each Fe-Al-Si phase, and the value displayed in "Length" on the toolbar is read to calculate T i Then, the calculated ΣT i and the corresponding surface length L0 of the base steel plate (length of the long side of the observation field: 100 μm), ΣT i / L0×100 (the projection ratio of the Fe—Al—Si phase) is determined.

[0043] The surface coverage of the Mg-containing phase is determined as follows: First, the length M of the Mg-containing phase present in the surface portion of the coating layer among the Mg-containing phases identified above is determined. i The sum of ΣM i (In Figure 3, ΣM i M = M + M + M is calculated using the "Analyze" function of the image analysis software "ImageJ." Specifically, when a horizontal line is drawn at both ends of each Mg-containing phase using the toolbar "Straight" in ImageJ, M is calculated by reading the value displayed in "Length" on the toolbar. ｉ Then, the calculated ΣM i and the corresponding surface length L0 of the base steel plate (length of the long side of the observation field: 100 μm), ΣM i / L0 x 100 (surface coverage of the Mg-containing phase) is determined.

[0044] The area ratio of the MgZn2 phase is measured from the element distribution image of the mapping image obtained for the above sample using the image analysis software "ImageJ." Specifically, the region in the element mapping containing 25 to 45 at% Mg and 50 to 75 at% Zn (Mg + Zn: 90 to 100 at%) is binarized using the "Binary" function of "ImageJ," and the area ratio is measured using the "Analyze" function.

[0045] The plating layer may be any plating layer having the above-mentioned chemical composition, an Fe—Al phase, an Fe—Al—Si phase, an Mg-containing phase, and / or an MgZn phase, and is not particularly limited, but may be, for example, an alloyed hot-dip plating layer.

[0046] [Preferable Chemical Composition of Base Steel Sheet] As described above, the present invention aims to provide a plated steel sheet having an Al-containing plating layer and exhibiting improved corrosion resistance and cold workability after painting. This objective is achieved by optimizing the chemical composition of the plating layer, controlling the thickness of the Fe—Al phase contained in the plating layer to within a range of 4 to 50 μm, and controlling the interface shape between the plating layer and the base steel sheet to satisfy the relationship (L − L) / L × 100 ≧ 3. Therefore, it is clear that the chemical composition of the base steel sheet itself is not an essential technical feature for achieving the objectives of the present invention. Preferred chemical compositions of the base steel sheet used in plated steel sheets according to embodiments of the present invention are described in detail below. However, these descriptions are intended merely as examples of preferred chemical compositions of the base steel sheet and are not intended to limit the present invention to those using base steel sheets having such specific chemical compositions.

[0047] In an embodiment of the present invention, for example, the base steel plate has, in mass %, C: 0.01 to 0.50%, Si: 0.001 to 3.000%, Mn: 0.10 to 3.00%, Al: 0.0002 to 2.000%, P: 0.100% or less, S: 0.1000% or less, N: 0.0100% or less, Nb: 0 to 0.15%, Ti: 0 to 0.15%, V: 0 to 0.15%, Mo: 0 to 1.0%, Cr: 0 to 1.0%, Cu: 0 to 1.0%, Ni: 0 to 1.0%, B: 0 to 0.0100%, W: 0 to 1.000%, Hf: 0 to 0.050%, It is preferable that the alloy has a chemical composition consisting of Mg: 0 to 0.050%, Zr: 0 to 0.050%, Ca: 0 to 0.010%, REM: 0 to 0.30%, Ir: 0 to 1.000%, and the balance: Fe and impurities. Each element will be described in more detail below.

[0048] [C: 0.01 to 0.50%] C is an element that inexpensively increases tensile strength and is an important element for controlling the strength of steel. To fully obtain this effect, the C content is preferably 0.01% or more. The C content may be 0.05% or more, 0.10% or more, 0.15% or more, 0.20% or more, 0.30% or more, or 0.35% or more. On the other hand, excessive C content may result in a decrease in elongation. For this reason, the C content is preferably 0.50% or less. The C content may be 0.45% or less or 0.40% or less.

[0049] [Si: 0.001 to 3.000%] Si acts as a deoxidizer and is an element that suppresses the precipitation of carbides during the cooling process during cold-rolled sheet annealing. To fully obtain this effect, the Si content is preferably 0.001% or more. The Si content may be 0.010% or more, 0.100% or more, or 0.200% or more. On the other hand, excessive Si content may increase the steel strength while decreasing elongation. For this reason, the Si content is preferably 3.000% or less. The Si content may be 2.500% or less, 2.000% or less, 1.500% or less, or 1.000% or less.

[0050] [Mn: 0.10 to 3.00%] Mn is an element that improves the hardenability of steel and is effective in increasing strength. To fully obtain this effect, the Mn content is preferably 0.10% or more. The Mn content may be 0.30% or more, 0.50% or more, 1.00% or more, or 1.30% or more. On the other hand, excessive Mn content may increase the steel strength but decrease the elongation. For this reason, the Mn content is preferably 3.00% or less. The Mn content may be 2.80% or less, 2.50% or less, or 2.00% or less.

[0051] [Al: 0.0002 to 2.000%] Al acts as a deoxidizer for steel and has the effect of improving the soundness of steel. To fully obtain this effect, the Al content is preferably 0.0002% or more. The Al content may be 0.001% or more, 0.010% or more, 0.050% or more, or 0.100% or more. On the other hand, excessive Al content may generate coarse Al oxides, reducing the elongation of the steel sheet. For this reason, the Al content is preferably 2.000% or less. The Al content may be 1.500% or less, 1.000% or less, 0.800% or less, or 0.500% or less.

[0052] [P: 0.100% or less] P is an element that segregates at grain boundaries and promotes embrittlement of steel. Since a lower P content is preferable, the ideal P content is 0%. However, excessive reduction in the P content may result in a significant increase in costs. Therefore, the P content may be 0.0001% or more, 0.001% or more, or 0.005% or more. On the other hand, excessive P content may result in embrittlement of steel due to grain boundary segregation, as described above. Therefore, the P content is preferably 0.100% or less. The P content may be 0.050% or less, 0.030% or less, or 0.010% or less.

[0053] [S: 0.1000% or less] S is an element that generates non-metallic inclusions such as MnS in steel, resulting in a decrease in the ductility of steel parts. Since a lower S content is preferable, ideally 0%. However, excessive reduction in the S content may result in a significant increase in costs. Therefore, the S content may be 0.0001% or more, 0.0002% or more, 0.0010% or more, or 0.0050% or more. On the other hand, excessive S content may cause cracks originating from non-metallic inclusions during cold forming. Therefore, the S content is preferably 0.1000% or less. The S content may be 0.0500% or less, 0.0200% or less, or 0.0100% or less.

[0054] [N: 0.0100% or less] N is an element that forms coarse nitrides in steel sheets and reduces the workability of the steel sheets. Since a lower N content is preferable, the ideal N content is 0%. However, excessive reduction in the N content may result in a significant increase in manufacturing costs. Therefore, the N content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, excessive N content may form coarse nitrides as described above, reducing the workability of the steel sheets. Therefore, the N content is preferably 0.0100% or less. The N content may be 0.0080% or less or 0.0050% or less.

[0055] The base steel sheet preferably has the basic chemical composition described above. Furthermore, the base steel sheet may contain, as necessary, one or more elements selected from the group consisting of Nb: 0-0.15%, Ti: 0-0.15%, V: 0-0.15%, Mo: 0-1.0%, Cr: 0-1.0%, Cu: 0-1.0%, Ni: 0-1.0%, B: 0-0.0100%, W: 0-1.000%, Hf: 0-0.050%, Mg: 0-0.050%, Zr: 0-0.050%, Ca: 0-0.010%, REM: 0-0.30%, and Ir: 0-1.000%, in place of a portion of the remaining Fe. Each of these elements may be 0.0001% or more, 0.0005% or more, 0.001% or more, or 0.01% or more.

[0056] The remainder of the base steel plate other than the above elements is composed of Fe and impurities. The impurities in the base steel plate are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ore and scrap, when the base steel plate is industrially manufactured.

[0057] The chemical composition of the base steel sheet may be measured by a common analytical method. For example, the chemical composition of the base steel sheet may be measured by first removing the plating layer by mechanical grinding, and then measuring the chips using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) in accordance with JIS G 1201:2014. Specifically, for example, a 35 mm square test piece may be obtained from the base steel sheet at approximately half the thickness, and the composition may be determined by measuring the test piece using a Shimadzu ICPS-8100 or similar measuring device under conditions based on a pre-created calibration curve. C and S, which cannot be measured by ICP-AES, may be measured using a combustion-infrared absorption method, N may be measured using an inert gas fusion-thermal conductivity method, and O may be measured using an inert gas fusion-non-dispersive infrared absorption method.

[0058] [Thickness of Base Steel Plate] The thickness of the base steel plate is not particularly limited, and may be, for example, 0.2 mm or more, 0.3 mm or more, 0.6 mm or more, 1.0 mm or more, or 2.0 mm or more. Similarly, the thickness of the base steel plate may be, for example, 6.0 mm or less, 5.0 mm or less, or 4.0 mm or less.

[0059] <Method for manufacturing plated steel sheet> Next, a preferred method for manufacturing a plated steel sheet according to an embodiment of the present invention will be described. The following description is intended to exemplify a characteristic method for manufacturing a plated steel sheet according to an embodiment of the present invention, but is not intended to limit the plated steel sheet to one manufactured by the manufacturing method described below.

[0060] The plated steel sheet according to an embodiment of the present invention can be manufactured by, for example, carrying out a casting process in which molten steel having an adjusted chemical composition is cast to form a steel billet, a hot rolling process in which the steel billet is hot-rolled to obtain a hot-rolled steel sheet, a coiling process in which the hot-rolled steel sheet is coiled, a cold rolling process in which the coiled hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet, a pretreatment process, an annealing process in which the pretreated cold-rolled steel sheet is annealed, a cooling process in which the annealed cold-rolled steel sheet is cooled, and a plating process in which a plating layer is formed on the obtained base steel sheet. Alternatively, the base steel sheet may be pickled after the hot rolling process without being coiled, and then directly subjected to the cold-rolling process. Each process will be described in detail below.

[0061] [Casting Step] The conditions for the casting step are not particularly limited. For example, after melting in a blast furnace or an electric furnace, various secondary smelting processes may be carried out, and then casting may be carried out by a conventional method such as continuous casting or ingot casting.

[0062] [Hot Rolling Process] A hot-rolled steel plate can be obtained by hot-rolling a cast steel slab. The hot rolling process is carried out by reheating the cast steel slab directly or after cooling it once, followed by hot rolling. When reheating is carried out, the heating temperature of the steel slab may be, for example, 1100 to 1250°C. In the hot rolling process, rough rolling and finish rolling are usually carried out. The temperature and reduction ratio of each rolling step can be appropriately determined depending on the desired metal structure and plate thickness. For example, the end temperature of finish rolling may be 900 to 1050°C, and the reduction ratio of finish rolling may be 10 to 50%.

[0063] [Coiling process] The hot-rolled steel sheet can be coiled at a predetermined temperature. The coiling temperature can be appropriately determined depending on the desired metal structure, etc., and may be, for example, 500 to 800°C. The hot-rolled steel sheet may be recoiled before or after coiling, and subjected to a predetermined heat treatment. Alternatively, the hot-rolled steel sheet may be pickled after the hot-rolling process and then subjected to the cold-rolling process described below, without performing the coiling process.

[0064] [Cold Rolling Step] After subjecting the hot-rolled steel sheet to pickling or the like, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. The reduction ratio of the cold rolling can be appropriately determined depending on the desired metal structure and sheet thickness, and may be, for example, 20 to 80%. After the cold rolling step, the sheet may be cooled to room temperature, for example, by air cooling.

[0065] [Pretreatment Step] Next, a predetermined pretreatment step may be performed before annealing the cold-rolled steel sheet. Such a pretreatment step may include a degreasing treatment. The degreasing treatment may include, for example, passing an electric current through the cold-rolled steel sheet in a solution having a pH of 8.0 or higher (electrolytic treatment). The current density during the current passing is 1.0 to 8.0 A / dm 2 The energization time may be 5 to 10 seconds.

[0066] [Annealing Step] Next, the obtained cold-rolled steel sheet is annealed. The annealing step involves heating the cold-rolled steel sheet to a temperature of 780 to 900°C in an atmosphere with a dew point of -10 to 10°C and holding the temperature for 10 to 300 seconds. By performing the annealing step under these conditions, the surface layer of the cold-rolled steel sheet can be adequately decarburized. In this case, the reaction between the coating layer and the base steel sheet is promoted during the alloying treatment in the subsequent coating step, i.e., the alloying rate can be increased. As a result, an interface shape with greater irregularities can be achieved, in which the interface length L between the coating layer and the base steel sheet and the corresponding surface length L0 of the base steel sheet satisfy the relationship (L - L0) / L0 × 100 ≥ 3, thereby significantly improving the cold workability of the coated steel sheet.

[0067] If the dew point is lower than −10°C, the annealing temperature is lower than 780°C, and / or the annealing time is shorter than 10 seconds, the decarburization of the surface layer of the cold-rolled steel sheet is insufficient, making it impossible to obtain a sufficient alloying rate during the alloying treatment of the coating layer. As a result, it becomes impossible to achieve an interface shape that satisfies the relationship (L − L) / L × 100 ≧ 3 between the coating layer and the base steel sheet. On the other hand, if the dew point is higher than 10°C, the heating temperature is higher than 900°C, and / or the annealing time is longer than 300 seconds, an outer oxide layer may form on the surface of the base steel sheet, resulting in reduced galvanic properties, or excessive decarburization may reduce the strength of the finally obtained coated steel sheet. The atmosphere in the annealing step may be a reducing atmosphere, more specifically, a reducing atmosphere containing nitrogen and hydrogen, for example, a reducing atmosphere of 1 to 10% hydrogen (e.g., 3% hydrogen and the balance nitrogen).

[0068] [Cooling Step] The cold-rolled steel sheet whose surface layer has been decarburized in the annealing step needs to be appropriately cooled in the subsequent cooling step to obtain a desired surface layer structure. Specifically, the cooling step includes cooling from the heating temperature (annealing temperature) in the annealing step to a controlled temperature of 500 to 750°C at an average cooling rate of 5°C / s or more. This will be described in detail below.

[0069] Typically, the annealed cold-rolled steel sheet is then cooled to a temperature below 500°C, for example, to a temperature of about 200°C, and then reheated and plated. However, this temperature history results in the transformation of the austenitized metal structure into a structure such as bainite or martensite in the annealing process. Therefore, in the subsequent plating process, the metal structure such as bainite or martensite is alloyed with the plating layer. However, because the alloying rate between these metal structures and the plating layer is relatively slow, the final plated steel sheet cannot achieve an interface shape that satisfies the relationship (L - L) / L × 100 ≥ 3 between the plating layer and the base steel sheet. Therefore, in the cooling process of the present manufacturing method, it is extremely important to immerse the metal structure of the cold-rolled steel sheet, whose surface layer has been decarburized in the annealing process, in a plating bath while still containing a large amount of austenite phase, thereby directly alloying the austenite phase with the plating layer. In this regard, in this cooling step, by cooling from the annealing temperature to a controlled temperature of 500 to 750°C at an average cooling rate of 5°C / s or more, the metal structure of the cold-rolled steel sheet can be maintained in a state containing a larger amount of austenite phase. As a result, in the subsequent plating step, it becomes possible to achieve an alloying rate sufficient to directly alloy the austenite phase with the plating layer and realize a desired interface shape.

[0070] While not intending to be bound by any particular theory, it is believed that increasing the alloying rate by combining decarburization and the austenite phase causes unevenness in the alloying rate between areas where austenite grain boundaries exist and areas where they do not, and that this unevenness in the alloying rate results in the formation of an uneven surface at the interface between the coating layer and the base steel sheet. If the controlled temperature is less than 500°C, the austenite phase transforms into bainite or martensite, making it impossible to achieve a sufficient alloying rate in the subsequent plating process. Furthermore, if the average cooling rate from the annealing temperature to the controlled temperature of 500 to 750°C is less than 5°C / s, the transformation to ferrite becomes significant, similarly making it impossible to achieve a sufficient alloying rate in the subsequent plating process. As a result, in either case, it becomes impossible to achieve an interface shape that satisfies the relationship (L - L) / L × 100 ≥ 3 between the coating layer and the base steel sheet. On the other hand, if the controlled temperature exceeds 750°C, the temperature becomes higher than that suitable for the subsequent plating process, and the desired coating layer may not be obtained. From the viewpoint of realizing an interface shape with greater irregularities, the higher the average cooling rate from the annealing temperature to the control temperature of 500 to 750°C, the more preferable, and for example, 15°C / s or more is preferable. Although the upper limit is not necessarily limited, the average cooling rate is preferably, for example, 30°C / s or less.

[0071] [Plating Step] Next, in the plating step, a plating layer is formed on at least one, preferably both, surfaces of a cold-rolled steel sheet (base steel sheet). More specifically, the plating step is carried out by immersing the cold-rolled steel sheet cooled to the above-mentioned controlled temperature in a plating bath (bath temperature: e.g., 680 to 750°C) having a predetermined chemical composition while maintaining a state in which the austenite phase is contained in a large amount, and then heat-treating the cold-rolled steel sheet at an alloying temperature of 680 to 750°C for 0.5 to 20 seconds. By performing the alloying treatment under these conditions, the plating layer is appropriately alloyed so that the Fe—Al phase has a thickness of 4 μm or more, and a sufficient alloying rate can be achieved based on the combination of decarburization and austenite phase. As a result, an interface shape with greater irregularities can be achieved in which the interface length L between the plating layer and the base steel sheet and the corresponding surface length L of the base steel sheet satisfy the relationship (L - L) / L × 100 ≥ 3, thereby significantly improving the cold workability of the plated steel sheet.

[0072] If the alloying temperature is lower than 680°C, the coating layer solidifies without sufficient alloying, resulting in a reduced Fe content in the coating layer and / or an inability to obtain the desired Fe—Al phase thickness. As a result, the corrosion resistance of the coated steel sheet after painting is reduced. Furthermore, if the alloying treatment time is shorter than 0.5 seconds, the coating layer is insufficiently alloyed, making it impossible to create an uneven shape at the interface between the coating layer and the base steel sheet and / or to obtain the desired Fe—Al phase thickness. As a result, the cold workability and / or corrosion resistance of the coated steel sheet after painting is reduced. On the other hand, if the alloying temperature is higher than 750°C or the alloying treatment time is longer than 20 seconds, excessive alloying of the coating layer occurs, resulting in a flatter interface with fewer unevenness, and the final coated steel sheet may not satisfy the relationship (L − L0) / L0 × 100 ≧ 3. In this case, the cold workability of the coated steel sheet is reduced. In order to ensure the desired alloying, the alloying treatment time is preferably set to 5 to 20 seconds.

[0073] The plating step is carried out by, for example, hot-dip plating. The plating step is not limited to hot-dip plating, and may be electroplating, vapor deposition plating, thermal spraying, cold spraying, or the like. Other conditions of the plating step may be appropriately set taking into consideration the thickness and coating weight of the plating layer. For example, a cold-rolled steel sheet is immersed in a plating bath, then pulled out, and immediately sprayed with N2 gas or air by a gas wiping method, followed by cooling. This allows the coating weight of the plating layer to be adjusted within a predetermined range, for example, within a range such that the thickness of the Fe—Al phase is 4 to 50 μm.

[0074] [Cooling after plating] Finally, the base steel sheet to which the plating layer is attached is cooled to obtain a plated steel sheet according to an embodiment of the present invention. The cooling after plating is not particularly limited and can be performed under any appropriate conditions known to those skilled in the art. For example, the cooling after plating can be performed at an average cooling rate of 10°C / s or more. The cooling stop temperature is also not particularly limited and may be set appropriately in the range of 100 to 350°C, for example.

[0075] According to this manufacturing method, it is possible to manufacture a plated steel sheet having a plating layer in which the chemical composition of the plating layer is optimized within a predetermined range, i.e., by mass, Fe: 20.0 to 55.0%, Mg: 0 to 10.0%, Si: 0 to 10.0%, and Al: 20.0% or more, the thickness of the Fe—Al phase contained in the plating layer is controlled within a range of 4 to 50 μm, and the interface shape between the plating layer and the base steel sheet is controlled to satisfy the relationship (L − L) / L × 100 ≧ 3. Therefore, due to the appropriately alloyed Fe—Al phase in the plating layer, sufficient corrosion resistance after painting can be ensured and cold workability can be improved. In addition, by controlling the interface shape between the plating layer and the base steel sheet to have a shape with greater irregularities, even when subjected to cold working such as bending, the hard plating layer can penetrate into the base steel sheet from the irregularities at the interface, deforming the base steel sheet as the cold working progresses. As a result, it is possible to significantly suppress the occurrence of powdering due to bending or the like, thereby significantly improving the cold workability of the plated steel sheet. In addition, by appropriately controlling the Si content in the coating layer, the Fe—Al—Si phase can be dispersed within the Fe—Al phase, thereby further improving the corrosion resistance of the plated steel sheet after painting. Furthermore, by appropriately controlling the Mg content, mainly in the coating layer, it is possible to increase the surface coverage of the coating layer by the Mg-containing phase, thereby significantly improving the chemical treatability of the plated steel sheet. Therefore, such plated steel sheets can achieve superior corrosion resistance and cold workability after painting compared to conventional plated steel sheets. This can contribute to industrial development by improving productivity in the use of plated steel sheets for automobiles and building materials.

[0076] The present invention will be described in more detail below with reference to examples, but the following examples are merely illustrative of the present invention and are not intended to limit the present invention in any way. It goes without saying that the present invention can be modified as desired without departing from the gist of the present invention.

[0077] In the following examples, plated steel sheets according to the embodiments of the present invention were produced under various conditions, and the properties of the produced plated steel sheets were investigated.

[0078] First, molten steel was cast by a continuous casting method to form a steel slab having a chemical composition consisting of, by mass%, C: 0.20%, Si: 0.012%, Mn: 1.30%, Al: 0.030%, P: 0.005%, S: 0.0020%, and N: 0.0030%, with the balance consisting of Fe and impurities. The steel slab was once cooled, reheated to 1200 °C, hot rolled, and then coiled at a temperature of 600 °C or less. Hot rolling was performed by performing rough rolling and finish rolling, with the finish rolling ending temperature being 900 to 1050 °C and the finish rolling reduction being 30%. Next, the obtained hot-rolled steel sheet was subjected to pickling and then cold-rolled at a reduction ratio of 50% to obtain a cold-rolled steel sheet having a thickness of 0.8 mm. Next, the obtained cold-rolled steel sheet was subjected to pickling at 5.0 A / dm 2 A pretreatment (degreasing treatment) was carried out by passing a current at a current density of 1000 kJ / min for 8 seconds.

[0079] Next, each cold-rolled steel sheet was cut into a size of 100 mm x 200 mm and then subjected to annealing treatment (annealing atmosphere: 3% hydrogen and balance nitrogen) under the conditions shown in Table 1. Next, the cut steel sheet samples were cooled from the annealing temperature to the control temperature at the average cooling rate shown in Table 1, and then immersed in a hot-dip galvanizing bath having a predetermined bath composition (galvanizing bath temperature: 680 to 750°C) and subjected to alloying treatment under the conditions shown in Table 1. After immersion in the galvanizing bath, the steel sheet samples were pulled out and subjected to N2 gas wiping to adjust the coating weight. Finally, the base steel sheet with the coating layer attached was cooled at an average cooling rate of 10°C / s or more to obtain coated steel sheets in which coating layers were formed on both sides of the base steel sheet.

[0080] The physical properties and characteristics of the resulting plated steel sheets were measured and evaluated by the following methods.

[0081] [Analysis of Chemical Composition of Plating Layer] The chemical composition of the plating layer was determined by immersing a sample cut to 30 mm × 30 mm in a 10% HCl aqueous solution containing 0.04% IBIT 710K (manufactured by Asahi Chemical Industry Co., Ltd.) as an inhibitor, pickling the plating layer, and then measuring the plating components dissolved in the aqueous solution by ICP emission spectroscopy. The results are shown in Table 1.

[0082] [Evaluation of Cold Workability] Cold workability was evaluated as follows. First, a plated steel sheet sample measuring 100 mm x 50 mm x 0.8 mm was subjected to a 90° bending test with R = 2 mm, then ultrasonically cleaned, and the sample mass was measured. The difference from the sample mass before the 90° bending test was measured as the amount of powdering, and the cold workability, particularly powdering resistance, was evaluated as follows: AAA: 6 mg or less AA: more than 6 to 12 mg A: more than 12 to 24 mg B: more than 24 mg

[0083] [Evaluation of Corrosion Resistance After Painting] The corrosion resistance after painting was evaluated as follows. First, a 50 mm x 100 mm sample of plated steel sheet was subjected to zinc phosphate treatment (SD5350 system: Nippon Paint Industrial Coating standard), then electrodeposition coating (PN110 Powernics Gray: Nippon Paint Industrial Coating standard) was performed to a thickness of 20 μm, and baking treatment was carried out at a temperature of 150°C for 20 minutes. Next, a cut reaching the base steel (base steel sheet) was introduced in the center of the sample. Next, a combined cyclic corrosion test according to JASO (M609-91) was carried out for 180 cycles, the paint film blister width was measured, and the corrosion resistance after painting was evaluated as follows. AAA: 2 mm or less AA: More than 2 to 3 mm A: More than 3 to 4 mm B: More than 4 mm

[0084] [Evaluation of Chemical Conversion Treatability] Chemical conversion treatability was evaluated as follows. First, a 50 mm x 100 mm sample of plated steel sheet was treated with zinc phosphate (SD5350 system: standard manufactured by Nippon Paint Industrial Coating Co., Ltd.) to form a chemical conversion coating. Next, the sample surface was observed using a secondary electron image from an SEM, and the area ratio of the portion where no chemical conversion coating was formed, commonly known as "clear," was measured. The chemical conversion treatability of the plated steel sheet was evaluated according to the area ratio of the clear area using the following evaluation criteria: AA: Clear area ratio 0-5% A: Clear area ratio more than 5-15% B: Clear area ratio more than 15%

[0085] Plated steel sheets that were rated AAA, AA, or A for cold workability and AAA, AA, or A for corrosion resistance after painting were evaluated as having improved corrosion resistance and cold workability after painting. The results are shown in Table 1.

[0086]

[0087]

[0088] Referring to Table 1, in Comparative Examples 30 and 31, the high Mg and Si contents in the coating layers presumably prevented a sufficient alloying rate from being achieved during the alloying treatment of the coating layers. As a result, the value of (L - L) / L × 100 at the interface with the base steel sheet was less than 3, i.e., the interface with the base steel sheet had a flatter shape with fewer irregularities, resulting in poor cold workability. In Comparative Example 32, the low annealing temperature presumably prevented insufficient decarburization of the surface layer of the cold-rolled steel sheet, preventing a sufficient alloying rate from being achieved during the alloying treatment of the coating layers. As a result, the value of (L - L) / L × 100 was less than 3, resulting in poor cold workability. In Comparative Example 33, the short annealing time presumably also prevented insufficient decarburization of the surface layer of the cold-rolled steel sheet, preventing a sufficient alloying rate from being achieved during the alloying treatment of the coating layers. As a result, the value of (L - L) / L × 100 was less than 3, resulting in poor cold workability. In Comparative Example 34, the dew point in the annealing process was low, which similarly resulted in insufficient decarburization of the surface layer of the cold-rolled steel sheet, and it is believed that a sufficient alloying rate could not be achieved during the alloying treatment of the coating layer. As a result, the value of (L - L) / L × 100 was less than 3, and cold workability was reduced. In Comparative Example 35, the average cooling rate from the annealing temperature to the control temperature of 500 to 750 °C was slow, which resulted in significant transformation from austenite to ferrite in the metal structure of the cold-rolled steel sheet, and it is believed that a sufficient alloying rate could not be achieved in the subsequent coating process. As a result, the value of (L - L) / L × 100 was less than 3, and cold workability was reduced.

[0089] In Comparative Examples 36 and 37, the controlled temperature in the annealing process was low, which likely led to significant transformation from austenite to bainite or martensite in the metal structure of the cold-rolled steel sheet, making it impossible to achieve a sufficient alloying rate in the subsequent plating process. As a result, the value of (L - L) / L × 100 was less than 3, resulting in poor cold workability. In Comparative Example 38, the alloying temperature of the coating layer was low, which likely led to solidification of the coating layer in an insufficient alloying state. As a result, the Fe content in the coating layer decreased, and the desired Fe-Al phase thickness could not be achieved, resulting in poor corrosion resistance after painting. In Comparative Example 39, the alloying temperature of the coating layer was high, which likely led to excessive alloying of the coating layer. As a result, the value of (L - L) / L × 100 was less than 3, which meant that the interface with the base steel sheet had a flatter shape with fewer irregularities, resulting in poor cold workability. In Comparative Example 40, the alloying treatment time of the coating layer was too short, resulting in insufficient alloying of the coating layer, and the interface between the coating layer and the base steel sheet could not be formed into an uneven shape. In other words, the value of (L - L0) / L0 x 100 was less than 3, and the desired Fe-Al phase thickness could not be obtained. As a result, the cold workability and post-painting corrosion resistance were deteriorated. In Comparative Example 41, the alloying treatment time of the coating layer was too long, which is thought to have caused excessive alloying of the coating layer. As a result, the value of (L - L0) / L0 x 100 was less than 3, resulting in deterioration of cold workability. In Comparative Example 42, the thickness of the Fe-Al phase was too thick, resulting in excessive hardening of the coating layer and therefore deterioration of the cold workability of the coated steel sheet.

[0090] In contrast, in all of the plated steel sheets according to the examples, the chemical composition of the coating layer was optimized within predetermined ranges, i.e., by mass: 20.0 to 55.0% Fe, 0 to 10.0% Mg, 0 to 10.0% Si, and 20.0% or more Al, the thickness of the Fe-Al phase contained in the coating layer was controlled within a range of 4 to 50 μm, and the interface shape between the coating layer and the base steel sheet was controlled to satisfy the relationship (L - L) / L × 100 ≥ 3. This significantly improved both the post-painting corrosion resistance and cold workability of the resulting plated steel sheets. In particular, in Examples 8 to 11, in which the (L - L) / L value was controlled to 5 or more, the cold workability was evaluated as AA, further improving cold workability. Similarly, in Examples 12 to 29, in which the (L - L) / L value was controlled to 7 or more, the cold workability was evaluated as AAA, further improving cold workability. In addition, the thickness of the Fe-Al phase is set to 12 μm or more, and ΣT i In Examples 14 to 29, in which / L0×100 was controlled to 1 or less (i.e., the projection rate of the Fe-Al-Si phase was 1% or less), the corrosion resistance after painting was evaluated as AAA, and very high corrosion resistance after painting was achieved. In addition, in Examples 8 to 11, in which the surface coverage of the Mg-containing phase was controlled to 20% or more, the chemical conversion treatability was evaluated as A, and similarly, in Examples 12 to 24 and 26 to 29, in which the surface coverage of the Mg-containing phase was controlled to 60% or more, the chemical conversion treatability was evaluated as AA, and very high chemical conversion treatability was achieved.

[0091] 1 Plated steel sheet 2 Base steel sheet 3 Plated layer 4 Fe-Al phase 5 Fe-Al-Si phase 6 Mg-containing phase L Interface length between plated layer and base steel sheet L0 Surface length of base steel sheet

Claims

1. A steel plate having a base steel sheet and a plating layer formed on a surface of the base steel sheet, The plating layer comprises, in mass %, Fe: 20.0 to 55.0%, Mg: 0-10.0%, Si: 0 to 10.0%, Zn: 0-30.0% and further comprising Ni: 0-1.000%, Ca: 0-4.000%, Sb: 0 to 0.500%, Pb: 0 to 0.500%, Cu: 0 to 1.000%, Sn: 0-1.000%, Ti: 0 to 1.000%, Cr: 0-1.000%, Nb: 0 to 1.000%, Zr: 0 to 1.000%, Mn: 0 to 1.000%, Mo: 0-1.000%, Ag: 0-1.000%, Li: 0 to 1.000%, La: 0 to 0.500%, Ce: 0-0.500%, B: 0 to 0.500%, Y: 0 to 0.500%, Sr: 0-0.500%, In: 0 to 0.500%, Co: 0 to 0.500%, Bi: 0-0.500%, P: 0 to 0.500%, W: 0 to 0.500%, and V: 0~0.500% Contains at least one of the following in a total amount of 5.000% or less, The balance has a chemical composition consisting of 20.0% or more Al and impurities, In the cross section of the plating layer, the interface length L between the plating layer and the base steel sheet and the surface length L of the base steel sheet 0 But (LL 0 ) / L 0 × 100≧3 is satisfied, The plated steel sheet is characterized in that the plated layer contains an Fe—Al phase, and the thickness of the Fe—Al phase is 4 to 50 μm.

2. The plated steel sheet according to claim 1, wherein 20≧(LL 0 ) / L 0 ×100≧3.

3. (LL 0 ) / L 0 The plated steel sheet according to claim 1 or 2, wherein x100≧5.

4. (LL 0 ) / L 0 The plated steel sheet according to claim 3, wherein x 100≧7.

5. The plated steel sheet according to claim 1 or 2, wherein the Mg content in the plated layer is 0.2% or more.

6. The chemical composition is, in mass %, Mg: 0.3 to 10.0%, and The plated steel sheet according to claim 1 or 2, characterized by containing Si: 0 to 1.0%.

7. The plated steel sheet according to claim 1 or 2, wherein the thickness of the Fe—Al phase is 12 to 50 μm.

8. In the cross section of the plating layer, the projected length T of the Fe—Al—Si phase in the plating layer i and the length L of the surface of the base steel plate 0 is ΣT i / L 0 The plated steel sheet according to claim 1 or 2, wherein x 100≦20 is satisfied.

9. ΣT i / L 0 The plated steel sheet according to claim 8, wherein x 100≦1.

10. The chemical composition contains, in mass%, Mg: 0.3 to 10.0%; the plating layer further includes an Mg-containing phase, 3. The plated steel sheet according to claim 1, wherein the Mg-containing phase has a surface coverage of 20 to 100% in a cross section of the plated layer.

11. The plated steel sheet according to claim 10, wherein the Mg-containing phase has a surface coverage of 60 to 100%.

12. The plated steel sheet according to claim 1 or 2, wherein the Mg content in the plated layer is 2.4% or less.

13. The plated steel sheet according to claim 1 or 2, wherein the Si content in the plated layer is 0.2% or more.

14. In the plating layer, MgZn 2 The plated steel sheet according to claim 1 or 2, wherein the area ratio of the phase is less than 10%.